uses MSHTML;
procedure TForm1.Button1Click(Sender: TObject);
begin
  WebBrowser1.Navigate('about:blank');
  while (WebBrowser1.ReadyState<>READYSTATE_COMPLETE) do
     Application.ProcessMessages;
  (WebBrowser1.Document as IHTMLDocument2).designMode := 'On';
end;

#include 
void __fastcall TForm1::Button1Click(TObject *Sender)
{
    CppWebBrowser1->Navigate(L"about:blank");
    while (CppWebBrowser1->ReadyState!=::READYSTATE_COMPLETE)
       Application->ProcessMessages();
    IHTMLDocument2 *pDoc;
    HRESULT hr = CppWebBrowser1->Document->QueryInterface(IID_IHTMLDocument2, (void**)&pDoc);
    if(hr == S_OK)
        pDoc->put_designMode(L"On");
    pDoc->Release();
}